In accordance with the Clean Air Act, the amendments made thereto, and numerous other municipal, state and Federal laws and/or regulations, large scale industrial processing facilities are required to perform compliance monitoring of their emissions to ensure designated pollutant levels are not exceeded. Compliance monitoring is typically required to be performed on a continuous basis, but occasionally is only required on a periodic basis. In addition, emission monitoring is also performed as a process control measure for such purposes as evaluating combustion efficiency or fine tuning an emission clean-up scheme. Consequently, emissions monitoring plays an important role in protecting the health and welfare of the environment and in the efficient use of our natural resources as used in industrial operations.
In most industrial process facilities, the exhaust stack is positioned adjacent an emission source, such as a boiler, in order to receive and direct the process exhaust upwardly into the atmosphere. In order to measure specific components of the emissions in the process exhaust, a sample line typically draws a portion of the exhaust from the exhaust stack and delivers it to a monitoring facility where sensors measure the amount of specific pollutants comprising the exhaust. In this configuration, the sample line is typically heated to maintain the temperature of the emissions within a desired range.
At the monitoring facility, any one of numerous types of sensors may be utilized to measure the gaseous components of the exhaust. Typically, the sensors are constructed to measure a particular gas of interest, or target gas, such as carbon monoxide (CO) or nitrogen oxide (such as NO or NO.sub.2, generically referred to hereinafter as NO.sub.x).
An example of such a sensor is a catalytic heat-flux sensor which includes two probes, an active and a reference, which are exposed to a gas sample extracted from the exhaust stream. At the tip of each probe is a precision temperature detector that changes resistance with changes in temperature, hereinafter referred to as a resistive temperature device (RTD). The active probe is coated with a catalyst which selectively promotes an exothermic reaction with the target gas so that an amount of heat directly proportional to the concentration of the target gas is generated. By measuring the amount of heat produced; the concentration of the target gas in the sample taken from the exhaust stack can be determined. This is typically done by measuring the differential resistance between the active probe and reference probe. Such catalytic heat-flux sensors are commercially available from numerous manufactures such as Advanced Sensor Devices, Inc., Mountain View, Calif., U.S.A.
Associated with each sensor, as a part of an emissions measuring system, is a circuit for reading the changes in resistance and for processing that information into an actual measurement value having the appropriate measurement unit such as parts per million (ppm). In FIG. 1, a well known measuring system 1 utilizing a catalytic heat sensor 2 is illustrated. As shown, the catalytic heat sensor 2 comprises an active RTD sensor 3a and a reference RTD sensor 3b. The active RTD sensor is coated with a catalyst that reacts with a target gas of interest, causing its temperature to rise when exposed to the target gas, and thereby, causing the resistance of the active RTD 3a to increase a proportional amount. Since the active and reference RTDs 3a, 3b are configured in parallel, an increase in resistance of the active RTD 3a causes the excitation signal from an oscillator 4 to be unequally divided between the active and reference RTDs 3a, 3b. Specifically, the current passing through the active RTD 3a and an active winding 5 differs from that passing through the reference RTD 3b and a reference winding 6. The different currents passing through the respective windings 5, causes a differential flux in the core of a transformer 7, which is detected by a sense winding 8. A current associated with the differential flux is converted by a conversion circuit 9 into a voltage signal which is proportional to the amount of gas detected by the catalytic heat sensor 2.
However, there are several inaccuracies built into such a circuit. For instance, transformers have an inherent non-linearity in their operation which results in varied performance throughout the range of currents that are applied to the transformer's windings. For example, a change in the differential current between active winding 5 and reference winding 6 does not produce an exactly corresponding change in the current in the sense winding 8 due to the non-linearity of the magnets of transformer 7. Moreover, the greater the current differential, the greater the effect of the non-linearity. This essentially reduces the range of valves for which the measuring system can accurately read.
In addition, the configuration shown in FIG. 1 utilizes a bridge circuit at the sensor in order to maintain a constant voltage across the active and reference RTD sensors 3a, 3b. Consequently, nulling must be performed in order to calibrate the active and reference RTD sensors 3a, 3b to account for manufacturing errors. Typically, this is accomplished by providing a direct current (d.c.) offset via an op amp (not shown) that is incorporated into the conversion circuit 9. This has proven to be an inaccurate and inefficient manner of nulling the sensor 2 because the practical implementation of a variable d.c. offset is inherently inaccurate, and further, because commercially available potentiometers lack sufficiently low temperature coefficients to accurately null the circuit over the normal range of operating temperatures.
Lastly, the electric components implementing the measuring system of FIG. 1 introduce drift into the circuits as a function of the change in environmental temperature and component aging. For example, the conversion 9 usually contains an amplifier whose gain changes with component valves. Thus, the output reading will change with changes in the gain, and therefore, producing an erroneous reading.
Hence, a heretofore unaddressed need exists in the industry for an emissions measuring system for use in emission monitoring that is capable of accurately measuring a wide range of concentration levels, with high sensitivity and low drift.